Hybrid integrated circuits used in automotive applications often employ surface-mounted circuit components. Such components are typically soldered to the surface of a circuit board to make electrical contact to a conductor formed on the circuit board. These components are generally enclosed within a protective outer casing which includes a heatsink that is maintained in contact with the circuit board. Both electricity and heat are conducted between the circuit component and the conductor through this heatsink. To conduct heat away from the conductor, the circuit board will typically be formed from a ceramic material in that ceramic is generally a better thermal conductor than other conventional circuit board materials, such as fiberglass. A substrate heatsink will typically be secured to the opposite surface of the circuit board, so as to provide a conduction path from the circuit board to the environment.
To promote both electrical and thermal contact with the circuit component, the conductor is typically formed to include a conductor pad to which the heatsink of the circuit component is soldered. The conductor pad is generally an enlarged area of the conductor, which serves to reduce the electrical and thermal resistance between the circuit component and the conductor. Conventionally, a single mass of solder is deposited on the conductor pad such that the entire surface interface between the component's heatsink and the conductor pad is occupied by the solder. The integrity of the electrical and thermal contact between the circuit component and the conductor pad relies not only on the amount of solder used to secure the circuit component to the conductor pad, but also on the quality of the solder joint formed by the solder. More particularly, the strength of the solder joint is critical to ensure that the circuit component will remain secured to the conductor pad to maintain both electrical and thermal contact. In addition, the uniformity of the solder joint ensures a low thermal and electrical resistance between the component and the conductor pad.
The strength and uniformity of the solder joint can be influenced by many factors. For instance, as the surface area of the solder joint between the component and the conductor pad increases, the strength of the solder joint increases. An increased surface area also corresponds to a larger conduction path between the component and the conductor pad, resulting in a lower resistance to electrical and thermal conduction between the component and the conductor pad. Thus, optimally, the size and shape of the conductor pad would provide the same surface area as that of the component's heatsink, and the entire interface area between the heatsink and the conductor pad would be occupied with a continuous, uniform layer of solder compound.
In addition, the potential for forming voids within the solder joint generally increases as the size of the circuit component increases. Specifically, a larger surface area occupied by the circuit component on the conductor pad corresponds to a decreased ability for flux gases produced during the solder process to escape from beneath the circuit component. While prior art soldering methods have been able to minimize the formation of solder joint voids in the past, current technologies which incorporate very large surface mounted circuit components in hybrid integrated circuits have sometimes resulted in the formation of voids within the solder joint between such components. Consequently, such solder joints are more prone to failure due to numerous thermal cycles and vibration, both of which are prevalent in automotive applications.
Thus, it would be desirable to provide a method for enhancing the solderability of a large surface-mounted circuit component to a conductor pad formed on a substrate, by preventing or at least substantially avoiding the formation of voids within the solder joint which adheres the circuit component to the conductor pad, such that the strength and durability of the solder joint is optimized as well as the electrical and thermal contact between the circuit component and the conductor pad.